Frequency modulation distortion correction system



Nov. 10, 1942. I R. .1. PIERACCI 2,301,907

FREQUENCY MODULATION DISTORTION CORRECTION SYSTEM Filed July 24, 1941 2Sheets-Sheet l l: in].

- IN KEN TOR. 305 149 jouylz mauled A rfamv; y

Nov. 10,1942. R J PIERACC] 2,301,907

FREQUENCY MODULATION DISTORTION CORRECTIQN SYSTEM Filed July 24, 1941 '2Sheets-Sheet 2 IIlVENTOR. (Rqgew Jooeya Me zqcc C,

Patented Nov. 10, 1942 FREQUENCY MODULATION DISTOBTION CORRECTION SYSTEMRoger Joseph Pieracci, Cedar Rapids, Iowa, ao-

signor to Collins Radio mpany, Cedar will.

Iowa, a corporation of Iowa Application July 24, 1941, Serial No.403,910

8 Claims. (Cl. 179-1715) My invention relates broadly to modulationsystems and more particularly to a frequency modulation distortioncorrection system.

One of the objects of my invention is to provide a system of frequencymodulation in which the distortion at wide phase angle deviations isminimized. The system of my invention applies particularly to phasemodulators in which the side bands are added to the carrier in phasequadrature. The operation of phase modulators in the production offrequency modulated signals is well known to the art. Present systemsuse a maximum phase shift of the order of 30 degrees at low audiofrequencies.

In systems using 30 degrees phase shift, analysis of the demodulatedfrequency modulation signal shows that distortions of the order of 7-8percent are present. At 60 degrees phase shift the distortion is of theorder'of 28 to 30 percent. This is due to the'non-linear relation of thephase shift angle and the amplitude of the side band voltage. It is theobject of the system of my invention to provide an approximately linearrelation between phase shift angle and side band amplitude for maximumphase shifts as high as 60 degrees or more.

Other and further objects of my invention reside in the provision of animproved circuit arrangement for frequency modulation systems, as setforth in the following specification by reference to the accompanyingdrawings in which:

Figure 1 shows vectorily the relation of the carrier and side bands inthe frequency modula tion distortion correction system of my invention;Fig. 2 shows graphically the completed relation of phase shift angle andside band amplitude in the frequency modulation distortion correctionsystem of my invention; Fig. 3 diagrammatically shows the frequencymodulation distortion correction circuit of my invention; and Fig. 4shows curve diagrams representing the voltage relations in the frequencymodulation distortion correction circuit of my invention.

Fig. 1 illustrates the mechanics of the system c of my invention byvector representation of the carrier and side bands. The side bands areadded at an angle of 90 degrees as shown in the figure and theiramplitude varied in accordance with the audio frequency. However, thecarrier is also amplitude modulated simultaneously at twice the audiofrequency in such a manner that approximately a linear relation obtainsbetween side band amplitude and angle of phase shift.

In Fig. 1, the side band amplitude BC is given by:

BC=sin pt (1) where is the audio modulating frequency BC 0 ==arc tan 27Efrom (1) and (2) sin pt K +K; cos 2 (4) If K1 and K: are properlychosen, it can be shown that an approximate linear relation between thephase shift angle 0 and side band amplitude sin t exists. Since (4) is atranscendental equation, the solution is not a simple matter. In thiscase, point-by-point solution is the best means of establishingoperating conditions. In an actual case, the maximum value of 0 (thelimiting value approaches 90 degrees) is selected and K1+K2 cos 2 tconsidered as a function of time Kf(t). 0 is then assumed to be linearwith sin t and Kflt) computed at intervals of the audio frequency cycle,so that this linearity obtains. From the resultant plotting of Kflt),the constants K1 and & may be evaluated. Actual plots show that themodulation of the-carrier very closely approximates a double audiofrequency cosine wave and is the reason for the selection of theexpression X1+K2 cos 2 t to represent the car 0 arc tan rier.

Fig. 2 shows the computed relation of phase shift angle 0 and side bandamplitude sin t for the case where maximum was chosen as 60 degrees andthe constants K1 and K2 0.765 and .188 respectively. The pointsapproximate a straight line very closely and negligible distortionshould result. The dotted curve shows the relation between 0 and sin twhen. the carrier is nnmoclulated. The solid line shows the relationshipof the phase shift angle 0 and the side band amplitude sin t whencorrected for distortion.

Fig. 3 shows the circuit arrangement of the system of my invention. Thespeech input cir- I cuit is indicated at I connected through audiofrequency transformer 2 to the speech amplifier 3. The speech amplifier3 comprises a balanced triode system constituted by tubes l and 5, asshown with their grid circuits connected to the secondary winding ofspeech input transformer 2, through the audio correction networkcomprising resistors 6 and i, and condensers l0 and II. The secondarywinding of speech inputtransformer 2 has its mid-tap 8 connected to thecathode circuits of the triodes |5 and to ground 9. The output of thespeech amplifier 3 is coupled through transformer I! with the balancedmodulator comprising tubes "-16. The

tubes [4 and I5 are operated without a direct current anode supply andreceive energy solely from ,the audio power delivered by transformer |2.

The circuit of the balanced modulator includes the balanced tank circuitl5 comprising inductance I! connected at opposite ends with the anodesof tubes l4 and I5 through coupling condensers 8 and I9. The inductanceI1 is tuned by tank circuit condensers 25 and 2| which are connected inbalanced relation with their adjacent capacity areas connected to thecathode circuits of tubes l4 and I5 and to ground indicated at 22. Theinductance H has its midpoint 23 connected through the audioshuntingchoke coil 24 with the cathode circuit of tubes l4 and I5. Theradio frequency side band energy developed in tank circuit I5 issupplied to the output system 25 leading to additional stages offrequency multiplication and amplification through conductor 25,condenser 21 and resistor 28 connected to ground 29. Radio frequencyenergy is excluded from the speed frequency amplifier circuit coupledthrough transformer |2 with the balanced modulator tubes |4|5 by theinclusion of the choke coils 30 and 3| between the anodes of tubes l4and i5 and the secondary winding of the speech frequency inputtransformer |2 as shown.

The oscillator for the transmitter is shown at 32 constituting a triodecontrolled by piezo-electric crystal 33 and having its output circuit 34connected to a phase shifting network comprising resistance 35, variablecondenser 35, variable condenser 31 and resistance 38. Connections aretaken from taps 39 and 45 in the phase shifting network to the controlgrids of the balanced modulator tubes |4-|5 and to the control grid 4|of the carrier wave amplifier tube 42, respectively. The phase shift ofthe between the grid voltages of the balanced modulator and carriertubes are plus and minus forty-five degrees from oscillator voltage.Thus the total phase angle between carrier and side band voltages isninety degrees. The connection taken to the control grid 4| of thecarrier wave amplifier tube 42 includes coupling condenser 43. The inputcircuit to the carrier frequency amplifier tube 42 iscompleted throughresistance 44 to ground 45 and from ground 45 to the cathode 41. Theoutput circuit of the carrier wave amplifier tube 42 includes plate 48which connects through coupling condenser 49 with the common loadimpedance H of the balanced modulator tank IS. The carrier is thus addedto the side bands and carried through conductor 25 leading to the outputsystem 25. The voltage for the plate 48 of the carrier wave amplifiertube 42 is obtained from the variable tap 55 on output resistor 5| inthe output circuit of wave form corrector tube 52. This anode voltage isamplitude modulated in accordance with voltage appearing across resistor5|. The anode voltage is passed through the series connected variableresistor 53 shunted by condenser 54 through the choke coil 55 forimpressing audio frequency voltage on plate 48 of tube 42. The auxiliarygrid 4|a in tube 42 connects to the lead from the wave form correctortube 52 intermediate the resistance condenser combination 53-54 andchoke coil 55. The value of resistance 53 controls the parameter K1 inthe formulas recited above while the position of the tap 50, onresistance 5| controls the parameter K: in the above formulas. The radiofrequency output of the carrier wave amplifier tube 42 is modulated atdouble audio frequency in accordance with the plate voltage. A by-passcondenser 45a connects across the plate potential supply circuit fortube 42 as indicated.

The manner of obtaining amplitude modulation of the carrier waveamplifier tube 42 constitutes the novel and important feature of myinvention. This amplitude modulation of th! carrier at twice the audiomodulation frequenc; is obtained by the coact-ion of a frequency double1tube 55 with the wave form corrector tube 51 heretofore referred to. Thefrequency double: tube 55 is a full wave rectifier including platelectrodes 51 and 58 and cathodes 59 and 59 The plate electrodes 51 and58 are connected t4 the output of the speech amplifier at the sec ondaryof transformer l2. The cathodes 59 an 55 are connected through loadresistor 5| am biasing resistor 52 to ground at 54. Resistor 5 isconnected across the potential +B at 5 through series resistor 52. Thevoltage devel oped across resistor 52 limits the operating angl of thediode rectifier 55 and aids in controllin the characteristic of theplate voltage curve a shown in B of Fig. 4. This source of potentis +Bwhich connects at 55 can be from a commo: source and supply all pointsin Fig. 3 marke +3. The cathode heating potential for all c the tubesmay be obtained from a commo source. The load resistor 5| is providedwith a adjustable tap 51 which connects to the contrl grid 58 of thewave form corrector tube 5 Proper bias is obtained for the wave form 001rector tube 52 through the series cathode resist 5 59 shunted bycondenser I0 and connected to path to ground indicated at H. Potentialf( the screen 14 of tube 52 is supplied through n sistor I3 from thesource at 55. The screen pl tential is by-passed to ground by condenser1 Referring to Fig. 4 it will be understood th: the wave form of thevoltage delivered by tl full wave rectifier tube 52 has high harmoncontent and must be passed through the circu of wave form corrector tube52 which constitut a wave form correction circuit that corrects t] waveform to within five percent of the tn double audio frequency. The tube52 functio: as a super-control remote cut-ofi tube Whii shapes the halfwave pulses from full wave reci her tube to the desired form. The supecontrol wave form corrector tube 52 is biased cut-off and the gridvoltage limited to the thres old of grid current on audio peaks by thecat ode resistor 59 and shunted condenser 10 he:

55 tofore described. In Fig. 4 the curves shown A represent the voltageoutput of tube 55 whi are applied to the control grid 58 of wave fOlcorrector tube 52.

The characteristics of the super-control we 60 form corrector tube 52when operated in i manner described are such that the wave to of theplate voltage and grid voltage are not al: but follow the generalcontour shown in B Fig. 4. The plate voltage (solid curve) appro 6 matesa true double frequency cosine wa Slight discontinuities at the pointsABCD n be minimized by reducing the operating angle the diode full waverectifier. This is acco plished by means of resistors 52 and 53 oper ingfrom' the plate supply voltage 55 of frequei doubler tube 55.

The system of distortion correction of my vention permits large anglesof phase shift the phase modulator with low distortion. '1

decreases the amount of phase multiplication quired for wide bandfrequency modulation and results in simpler apparatus and a. reductionof noise due-to random disturbances in the phase modulator circuit.Present systems use 30 degrees phase shift and a multiplication of 4000to 5000. quency when the phase is modulated to the 30 degree maximum is'l-8%. Application of this system of correction to this circuit wouldreduce the distortion to 1-1.5 percent. The latter figure is based onexperimental data.

If a system using 60 degree maximum phase shift is used, the phasemultiplication can be reduced by half to 2000 to 2500. Thus the randomnoise should be reduced 6 db. The distortion without the correctionsystem would be 28-30% and with correction 2 to 3 percent. These latterfigures are also based on preliminary experimental data, and couldprobably be bettered by a more accurate synthesis of the doublefrequency audio voltage referred to above.

While I have described my invention in one of its preferred embodiments,I desire that it be understood that modifications may be made and Iintend no limitations upon my invention other than may be imposed by thescope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is as follows:

1. In a frequency modulated signaling system a radio frequencyoscillator, a carrier wave emplifier having a cathode, an anode, acontrol grid electrode and a screen grid electrode, means for impressingunmodulated radio frequency energy from the oscillator which is shiftedin phase at a fixed value of 45 degrees leading the oscillator voltageupon the control grid electrode, means for impressing a doubled audiofrequency modulation simultaneously upon said screen grid-electrode andsaid anode, and means connected with the output of said carrier waveamplifier subject to the composite influence of a basic audio frequencymodulated side band voltage and a carrier voltage modulated at twice theaudio frequency.

2. In a'frequency modulated signaling system an audio frequencymodulation circuit, a balanced amplifier connected with said audiofrequency modulation circuit, a balanced modulator having symmetricallyarranged output circuits and a common input circuit, a tuned tankcircuit connected with the output of said balanced modulator, a carrierwave amplifier having a pair of input circuits, means connected betweensaid audio frequency modulation circuit and one of the input circuits ofsaid carrier wave amplifier for impressing a doubled audio frequencymodulation upon said carrier wave amplifier, an oscillator, means forapplying energy from said oscilltor to the input circuit of saidbalanced modulator at a fixed phase difference lagging the oscillator by45 degrees, the input voltage at said carrier wave amplifier leading theoscillator voltage by 45 degrees whereby the net difierence at the inputcircuits of the said balanced modulator and the said carrier waveamplifier is 90 degrees, and a connection between the output of saidcarrier wave amplifier and said tuned tank circuit, whereby said tankcircuit is subjected to the composite effect of said balanced modulatoroutput and said carrier wave amplifier output.

3. In a frequency modulated signaling system an audio frequencymodulation circuit, a balanced amplifier connected with said audiofrequency modulation circuit, a balanced modulator The distortion atlowest audio ire-- cuit connected'with the output of said balancedmodulator, a carrier wave amplifier having a pair of input circuits,means connected between said audio frequency modulation circuit and oneof the input circuits of said carrier wave amplifier for impressing adoubled audio frequency modulation upon said carrier wave amplifier,means for correcting the wave form characteristic of said doubled audiofrequency modulation, an oscillator, means for applying energy from saidoscillator to the input circuit of said balanced modulator at a fixedphase difference lagging the oscillator by 45 degrees, the input voltageat said carrier wave amplifier leading the oscillator voltage by 45degrees whereby the net difference at the input circuit of said balancedmodulator and said carrier wave amplifier is degrees, and a connectionbetween the output of said carrier wave amplifier and said tuned tankcircuit, whereby said tank, circuit is subjected to the composite effectof said balanced modulator output and said carrier wave amplifieroutput.

4. In a frequency modulated signaling system an audio frequencymodulation circuit, a balanced amplifier connected with said audiofrequency modulation circuit, a balanced modulator having symmetricallyarranged output circuits and a common input circuit, a tuned tankcircuit connected with the output of said balanced modulator, a carrierwave amplifier having a pair of input circuits, means connected betweensaid audlo frequency modulation circuit and one of the input circuits ofsaid carrier wave amplifier for impressing a doubled audio frequencymodulation upon said carrier wave amplifier, an oscillator, means forapplying energy from said oscillator to the input circuit of saidbalanced modulator at a fixed phase difference lagging the oscillator by45 degrees, the input voltage at said carrier wave amplifier leading theoscillator voltage by 45 degrees whereby the net difference at the inputcircuits of said balanced modulator and said carrier wave amplifier is90 degrees, and a connection between the output of said carrier waveamplifier and said tuned tank circuit, whereby said tank circuits issubjected to the composite eiiect of said balanced modulator output andsaid carrier wave amplifier output, and a condenser and resistanceconnected in parallel with respect to each other and disposed in seriesbetween said means for correcting the wave form characteristic of saiddoubled audio frequency modulation and said carrier wave amplifier.

5. In a frequency modulated signaling system an audio frequencymodulation circuit, a balanced amplifier connected with said audiofrequency modulation circuit, a balanced modulator having symmetricallyarranged output circuits and a common input circuit, a tuned tankcircuit connected with the output of said balanced modulator, a carrierwave amplifier having a pair of input circuits, means connected betweensaid audio frequency modulation circuit and one of the input circuits ofsaid carrier wave amplifier for impressing an audio frequency modulationat twice the frequency of said first mentioned audio frequency upon saidcarrier wave amplifier, an oscillator, means for applying energy fromsaid oscillator to the input circuit of said balanced modulator at afixed phase difference lagging the oscillator by 45 degrees, the inputvoltage at said carrier wave amplifier leading the oscillator voltage by45 degrees whereby the net difference at the input circuit of said.carrier wave amplifier is 90 degrees, and a connection between theoutput of said carrier wave amplifier and said tuned tank circuit,whereby said tank circuit is subjected to the composite effect of saidbalanced modulator output and said carrier wave amplifier output.

- 6. The method of maintaining a linear relation between phase shiftangle and amplitude of side band voltage in frequency modulator systemswhich comprises compositely modulating a carrier wave amplifier byadding the voltages ofthe side band frequencies at an angle ofapproximately 90, varying their amplitude in accordance with an audiomodulating frequency deriving a second modulating signal from said audiomodulating frequency at double the said audio modulating frequency andsimultaneously subjecting the carrier wave amplifier to modulation bysaid second modulating signal in such manner that approximately a linearrelation obtains between side band amplitude and angle of phase shift.

"I. The method of minimizing distortion at wide phase deviations infrequency modulation systems which comprises simultaneously modulatingthe output voltage of a. carrier wave amplifier system and the side-bandvoltage of a balanced modulator system, the balanced modulator systembeing modulated by an audio frequency signal and the-carrier waveamplifier system being modulated simultaneously by a second signal whichis derived from the aforesaid audio frequency signal, the second signalbeing so derived that it is always twice the value of the signalingfrequency.

8. The method of eifecting an approximately linear relation between.phase shift angle and side-band amplitude in frequency modulationsystems which comprises compositely modulating the output voltage of acarrier wave amplifier system and the side-band voltage of a balancedmodulator system, the balanced modulator system being modulated by anaudio frequency signal and the carrier wave amplifier system beingmodulated simultaneously by a second signal which is derived from theaforesaid audio frequency signal, the second signal being so derivedthat it is always twice the value of the signaling frequency.

ROGER JOSEPH PERACCI.

CERTIFICATE OF CCRRECI'IORU Patent No. 2,501,907. November 10, 1&2.

' ROGER JOSEPH PIERACGI.

It is hereby certified thalt error appears in the prin ted specificationof the above numbered pzitent requiring correction as followsz Paige 1,first column, line 5 for"completed read "computedand that the SaidLetters Patent shouldbe reed with 131115 correction therein that theSame may conform to the record of the case in the Patent 'Office.

Signed and sealed this 12th day of Janu ary, ,A'. I 1&5.

Henry- Van Arsdale (Se&l) Acting Commissioner of Patents,

